There are various types of systems which automatically answer an incoming call and maintain a telephone line connection which are broadly defined as automatic telephone answering systems. These systems include, for example, automatic telephone message recording equipment, message recording systems having remote access message playback, automatic telephone call forwarding equipment, automatic dial-up alarm receiving equipment, automatic answering data entry systems, and other telephone devices used in association with modem equipment. Each of these systems typically include some means for establishing a connection to the telephone line and for disconnecting from the telephone line.
Disconnection from a telephone line is typically accomplished by providing a telephone tone detector which causes the disconnection of the telephone line in response to, for example, the detection of a dial tone generated after the calling party hangs up. Difficulties arise, however, in a message recording system having remote access playback capabilities. For example, a dial tone may be present during the recording interval of a message and, as a result, the dial tone may be recorded. When a remote access connection is established and the message is played back, the dial tone detector may detect the recorded dial tone and cause the inadvertent disconnection of the telephone line.
Accordingly, it would be desirable to have a simple and cost effective apparatus for detecting the source of a telephone tone signal that overcomes the disadvantages described above.